1. Field of the Invention
My invention relates to the preparation of nitrilotriacetonitrile, which is particularly useful in the formation, by hydrolysis, of nitrilotriacetic acid of particular value as a chelating or complexing agent. More particularly, my invention relates to the preparation of methylene-bis-iminodiacetonitrile and to the preparation of nitrilotriacetonitrile from the methylene-bis-iminodiacetonitrile and also to the preparation of nitrilotriacetic acid directly from the methylene-bis-iminodiacetonitrile.
2. Description of the Prior Art
In alkaline processes for the preparation of nitrilotriacetic acid salts, the formation of the intermediate nitrile and its hydrolysis to the nitrilotriacetic acid salt occur simultaneously. Alkaline processes include the classical carboxylation of ammonia with sodium cyanide and formaldehyde and processes in which partially carboxymethylated derivates such as glycine and iminodiacetic acid are used. Thus the carboxymethylation of glycine with sodium cyanide and formaldehyde or with glycolonitrile has been described. These processes, however, are intrinsically inferior to acid processes because side reactions involving formaldehyde and ammonia lead to the formation of substantial quantities of inert materials.
In acid processes, the nitrilotriacetonitrile produced may be isolated and then hydrolyzed to the nitrilotriacetic acid. Such processes include the preparation of the nitrile by the reaction of aminoacetonitrile with formaldehyde and an excess of hydrogen cyanide, as described in U.S. Pat. No. 2,405,966 to Loder; the addition of sodium cyanide to a hydrochloric acid solution of an ammonium salt and formaldehyde, as described in U.S. Pat. No. 2,205,995 to Ulrich and Ploetz; the addition of ammonia to a mixture of acid-stabilized formaldehyde and liquid hydrogen cyanide, as described in U.S. Pat. No. 2,855,428 to Singer and Weisberg; and the addition of hexamethylenetetramine to an acid-stabilized mixture of liquid hydrogen cyanide and formaldehyde, as described in U.S. Pat. No. 3,061,628 to Singer and Weisberg.
The acid processes, however, are characterized either by low yields and recycle problems caused by byproducts, e.g. inorganic salts, as in Loder and in Ulrich and Ploetz or by hazardous and inherently limited operating conditions as in the Singer and Weisberg processes. In these latter processes, a mass of hydrogen cyanide in excess is used in a closed reaction zone, i.e. a container or reaction pot, presenting the hazards of confining a highly toxic, flammable substance in a potentially explosive reaction. Moreover, the fixed volume of the container inherently limits rates of addition of reactants.